easy way: use 2 colors and mix them together, for example blue and orange, don't use black, dark blue is your darkest colour;
more hard way: take more wide palette, for example to our orange we can add more yellow in lights and more reds in dark points.
hardest way: use many colors and mix them in the one picture - usually it's a hard and long time process of many experiments at start stage of picture. Try make interesting colors in thumbnail and after that give details. I say again - do not use black in the sky (except star-sky), lights going to yellow and after white, darks going to red and after darkblue and violet. Hardest thing here is to find ballance